Generally, in the production of steel products, molten iron, which is used as a raw material of steel products, is produced in a stage prior to the final refinement for adjusting the chemical composition of steel products, and is subjected to purpose-specific refining, thereby obtaining the final product of steel material.
Examples of a starting material used in molten iron production include various iron-containing materials such as iron ore, iron oxide dust that is generated during refinement, reduced iron that is produced using iron ore as a starting material, scrap iron, etc. Basically, any iron-containing material can be used as a starting material of molten iron; however, in many cases, the kind of the starting material used and the production process employed depend on economic efficiency.
Although there are various molten iron production processes, from the viewpoint of the main starting material, blast furnace-LD converter processes in which iron ore is used as the main starting material are predominate in the world, followed by electric arc furnace processes in which scrap iron is used as the main starting material.
In a blast furnace process, carbon-saturated molten iron is obtained by using iron ore as the main starting material and coke as a reductant, and reducing the iron ore by blowing high-temperature heated air to it. However, the blast furnace process requires sintering equipment and a coke oven for pre-treating iron ore, which is the main starting material, and coal, which is a raw material of coke, which is used as a reductant, thus increasing capital-investment expenses. From a rational economic viewpoint, such a blast furnace process is said to be applicable only to a large-scale steel works (annual output of not less than 3 million tons). The blast furnace process is therefore not suitable unless large-scale production is required.
Of iron making processes using iron ore as a starting material, conventionally known processes used when the required output is not so large as the output from blast furnace processes include fluid bed reduction processes, and reduced iron production processes using natural gas. However, all of these processes are for yielding solid iron, and therefore require the step of melting the resulting solid iron. In general, the solid iron is used as a secondary material in an electric furnace or a LD converter, and melted therein.
In order to solve these problems, novel molten iron-manufacturing processes referred to as DIOS (Direct Iron Ore Smelting Reduction Process) or FINEX, in which molten iron is directly produced using iron ore as a starting material, have been developed. In these processes, a mixture that contains a carbon source in an amount required for the reduction of iron oxide is formed using iron oxide, such as iron ore, refined dust, etc., as the main starting material; or a method in which a carbon source is added in an amount required for reduction and heat generation to previously prepared molten iron while blowing pure oxygen gas into the molten iron at a speed equal to or faster than the speed of sound, to thereby combust carbon in the molten iron, and the molten iron is heated by the heat generated by combustion is employed. The reducing reaction of iron oxide is an endothermic reaction; if heat is not supplied from the exterior, the temperature decreases, stopping the reducing reaction and solidifying the molten iron, thus impeding the object of producing molten iron. Therefore, a large amount of carbon must be added to the molten iron to constantly maintain a substantially carbon-saturated condition, and pure oxygen gas is blown into the molten iron to combust a carbon element in the molten iron, so that heat required for the reducing reaction can be constantly supplied, thus maintaining a liquid state.
However, this method has a disadvantage in that considerable amounts of molten iron splashes are scattered out of the system as iron oxide dust, together with an exhaust gas. This dust generation causes considerable loss, such as sensible heat loss from dust, iron yield loss due to dust, expense for recycling the dust, etc. The phenomenon of bursting CO bubbles that are generated by the combustion reaction of carbon in the molten iron and pure oxygen gas is usually called bubble burst; and the dust generated by bubble burst is called bubble burst dust. The bubble burst phenomenon inevitably occurs when carbon in molten iron is combusted by oxygen gas. This is a significant problem to be solved, but solving the problem would be difficult.
An electric arc furnace process is a process in which molten iron is produced by melting scrap iron by using electric arc heating using a graphite electrode. Generally, in the electric arc furnace process, since the content of nitrogen in the obtained molten iron is as high as over 100 ppm, the resulting steel material is hard. Accordingly, this process cannot be used when a low nitrogen content is desired in view of the properties of steel material. Further, the electric arc furnace process has disadvantages in that a great deal of electric power consumption increases costs, the instability of the arc causes heat loss, etc.
To solve these problems, a process for melting cold iron sources has been developed and employed. This process involves adding scrap iron to previously prepared carbon-saturated molten iron, which is referred to as a molten seed, using a conventional LD converter, and blowing pure oxygen gas at supersonic speeds from above while adding pulverized coal from the bottom of the furnace, thereby heating and melting scrap iron using the heat of the combustion reaction of carbon in the molten iron. However, the generation of bubble burst dust cannot be sufficiently inhibited even in the process for melting cold iron sources.
As a means for heating molten iron, electric arc heating using a graphite electrode, which is commonly performed in an electric arc furnace, and a method of combustion using oxygen gas, a carbon element or a silicon element contained in pig iron that is obtained by a blast furnace process are widely used. Examples of heating methods used in the extremely limited processes include, although there is a limitation such that the methods are effective only when a decarbonization reaction is performed in a vacuum degassing apparatus, a method in which CO gas generated by a decarbonization reaction is combusted through the blowing of oxygen gas (RH-KTB process), and a method in which a fuel gas and a combustion-supporting gas are blown into a vacuum chamber (RH-MFB process). Also, there are particular heating methods, such as plasma heating, electric induction heating, etc.; however, they are only used with a molten steel distribution apparatus called a tundish, which is used in the casting step, for the purpose of maintaining the desired temperature when the temperature of molten steel is lower than the target temperature. Thus, they are not used as heating methods for general refining (see, for example, Patent Literatures 1 and 2).
These particular heating methods are used only for small-scale heating, i.e., for heating a target refining vessel, or for raising the temperature of molten steel to the target temperature when the temperature is lower than the target temperature. Accordingly, RH-KTB and RH-MFB methods are used only for small-scale heating in a vacuum degassing step, which is the final step of refining. Thus, since the refinement of molten iron requires a large amount of heat, only very limited means can be used as a method for supplying heat in the refinement of molten iron.
Pig iron obtained by a blast furnace process contains carbon, silicon, phosphorus, manganese, and like exothermic elements that are combusted by oxygen gas, and possesses a large quantity of sensible heat because it has a temperature as high as about 1500° C. These two points are the sources of heat used in converter refining in the blast furnace-LD converter process. Accordingly, the thermal limit of the blast furnace-LD converter process is determined by the amount of the resulting pig iron, the temperature of the pig iron, and the amount of the element (e.g., carbon, silicon, phosphorus, manganese, etc.) that generates heat via the reaction with oxygen gas, the element being contained in the pig iron.
From the viewpoint of heating methods, a currently-employed process for melting cold iron sources, and the aforementioned DIOS and FINEX are an extension of the technical idea of heating used in a blast furnace-LD converter process. Specifically, they are processes for producing molten iron, wherein a carbon source is added into molten iron, and melted to a substantially saturated state; pure oxygen gas is blown into the molten iron to combust the carbon in the molten iron; and scrap iron is melted using the heat generated by combustion, or heat required for the reduction/melting of iron oxide is supplied (for example, see Patent Literature 3). However, as long as the processes are based on this technical idea, the problem of generating bubble burst dust as described above cannot be solved.
The present patent aims to develop a novel means for efficiently supplying a large amount of heat, thus solving the unsolved problems of conventional molten iron manufacturing processes.